1. Field of the Invention
This document relates to a backlight unit and, more particularly, to a backlight unit which can reduce vibration and noise of a bottom cover and a liquid crystal display using the same.
2. Description of the Related Art
The liquid crystal display (LCD) has been increasingly used in a wide variety applications because of the associated characteristics such as light-weight, thinness, and low power consumption during driving. Following this trend of increasing use, the liquid crystal display has employed in such devices as office automation devices, and audio/video devices. An LCD displays desired images on a screen by controlling the intensity of a light beam applied to a plurality of control switches arranged in matrix.
Liquid crystal displays are not self-luminous and therefore LCD devices require a light source such as a backlight.
Backlight units for the liquid crystal display can be broadly classified as either direct lighting types or an edge lighting types. In the edge lighting type, a light source is disposed at the outer block of a flat panel, and light emitted from the light source is incident on the entire surface of a liquid crystal display panel using a transparent light guide plate. In the direct lighting type, a light source is disposed on the rear surface of a liquid crystal display panel, and the entire surface of the liquid crystal display panel is directly radiated. The direct lighting type is advantageous when compared with the edge light type in that it can deliver increased luminance and can distribute light unto a wide light-emitting surface since several light sources are used. In the case of an LCD TV employing a large-sized liquid crystal display panel, the backlight unit of the direct lighting type is generally adopted.
As shown in FIGS. 1 and 2, the backlight unit of a direct lighting type of the related art includes, a plurality of lamps 20 for illuminating light to a liquid crystal display panel (not shown), lamp guides 34 for supporting the lamps 20, a bottom cover 10 for accommodating the lamps 20 and the lamp guides 34, a diffusion sheet (not shown) disposed to cover the entire surface of the bottom cover 10 and configured to spread light incident from the plurality of lamps, and a plurality of optical sheets (not shown) stacked over the diffusion sheet and functioning to vertically raise a light travel path toward the liquid crystal display panel.
The bottom cover 10 is equipped with a bottom surface, an inclined face that extends at an angle away which from the bottom surface and then extends, and a wing face, which is extends at an angle from the inclined face. The bottom cover 10 accommodates the lamps 20, and the lamp guides 34 for supporting the lamps 20. Each of the lamp guides 34 includes a lamp holder 32 and a frame member 30. The lamp holder 32 is supported by the frame member 30 that is coupled to the bottom surface of the bottom cover 10. The lamp holder 32 and frame member function to fix the lamps 20 to the bottom cover 10. A diffusion sheet support 40 is coupled to the frame member 30 to prevent the diffusion sheet from sagging due to heat of the lamps 20. A reflection sheet 5 is attached to the bottom surface and to the inclined face of the bottom cover 10. The reflection sheet 5 functions to reflect light that travels from the plurality of lamps 20 toward the bottom cover 10, redirecting the reflected light toward the liquid crystal display panel to thus improve the efficiency of light illuminated on the liquid crystal display panel.
The lamp 20 generally employs a cold cathode fluorescent lamp (hereinafter referred to as “CCFL”) or an external electrode fluorescent lamp (hereinafter referred to as “EEFL”). A high voltage AC signal generated through an inverter (not shown) is applied to the lamp 20. A high voltage-side electrode of the lamp 20 is connected to the inverter through a lamp wire (not shown), and a low voltage-side electrode of the lamp 20 is grounded. The lamp 20 emits light when supplied intermittently with the output signal of the inverter through the lamp wire, as shown in FIG. 3. The duty ratio of the output signal of the inverter is controlled in response to a burst dimming signal generated from a dimming circuit (not shown). The duty ratio of the output signal of the inverter is Ton×100/(Ton+Toff), where ‘Ton’ denotes a turn-on period of the lamp 20 and ‘Toff’ denotes a turn-off period of the lamp 20. The duty ratio of the output signal determines the luminance of the lamp 20.
However, when the lamps 20 are operated based on the output signal of the inverter as described above, an AC signal of a high voltage is intermittently applied to the high voltage-side electrodes of the lamps 20, so that the lamps 20 are subject to micro vibration due to a periodical variation in the supplied energy, as shown in FIG. 4. The lamps 20 are fixed to the bottom cover 10 through the lamp holder and, therefore, the vibration of the lamps 20 is transferred to the bottom cover 10 substantially unchanged. The vibration of the bottom cover 10 generates noise as shown in FIG. 5. The generated noise is greater on the front side of the liquid crystal display having an exposed liquid crystal display panel than is the generated noise on the rear side of the liquid crystal display that is fully sealed by the back cover. Further, the problem of generation of noise due to vibration of the bottom cover 10 increases as a gap between the lamps 20 and the bottom cover 10 is reduced to produce a slim, wide-screen liquid crystal display.